1. Field of the Invention
The invention lies in the field of the packaging technology and concerns a strapping installation according to the generic part of the independent claim. The strapping installation serves the purpose of strapping objects, in particular of essentially parallelepipedic objects, e.g. of stacks of printed products such as newspapers, periodicals, etc. The installation particularly serves the purpose of essentially simultaneous strapping in two directions at a right angle to each other (cross-strapping). The strap used for strapping is tape-like (strapping tape), or string- or wire-like, and it is preferably stiff enough to be moved by pushing.
2. Background of the Invention
For automatic strapping, objects to be strapped are conveyed in succession on a bearing surface to a strapping position where each one is positioned and strapped, and from where they are removed again in succession. The strap is supplied to the strapping position, e.g. from a supply reel located below the bearing surface. Usually the strap is formed into a loop which is larger than the strapping to be achieved and which has a position from where it can be moved to encircle the object. By retracting the strap the loop is then placed around the object and tightened and usually it is then fastened beneath the object and severed from further supplied strap.
Preparing a loop for longitudinal strapping (strapping direction parallel to the conveying direction) is less simple than preparing a loop for transverse strapping (strapping direction transverse to the conveying direction), because in its final position, the longitudinal loop obstructs the supply route of the object to be strapped. Therefore, the loop cannot take up this position before the object is positioned in the strapping position. State of the art technology includes various methods for establishing a longitudinal loop such that the object to be strapped can still be supplied to the strapping position after, or preferably during loop preparation.
One such method is described in the publication EP-1207107, which proposes, for loop preparation, a groove-shaped loop channel integrated in the bearing surface. Thus the loop is prepared in a horizontal plane below the bearing surface where it does not obstruct conveyance of the object to be strapped. As soon as the object is positioned in the strapping position, the loop is lifted from the loop channel by a loop moving means and is brought into a vertical position in which it encircles the object to be strapped. From this position, the loop is placed on the object and tightened by strap retraction. The loop moving means e.g. comprises grippers positioned in the area of the groove-shaped channel prior to the loop motion and being moved along suitable tracks together with the loop for bringing the latter to its final position. As the grippers have a small weight they can be accelerated fast.
A further method is described in the publication U.S. Pat. No. 5,078,057 (or DE-4100276, Signode Corp.). This publication discloses essentially simultaneous longitudinal and transverse strapping (cross strapping). A loop channel is designated to each strapping, wherein the longitudinal channel rises arch-like above the bearing surface on one side of the object to be strapped. The upper part of the arch vaults above the strapping position. The channel does not obstruct the supply route to the strapping position and yet it extends partly above the object positioned for strapping. Such loop channel design makes it necessary, that the loop channel, at least for a tape-like strap, is not only bent but also twisted, which renders the loop channel to be a rather complicated and costly part of machinery.